Method of removal profile modulation in cmp pads

ABSTRACT

A polishing pad includes a plurality of polishing surfaces, a first group of the polishing surfaces made of a first material having a first coefficient of friction and a second group of the polishing surfaces made of a second material having a second coefficient of friction. The first and second groups of polishing surfaces may be arranged over the polishing pad so as to provide a non-planar material removal profile. The polishing surface layout may be designed by evaluating a material removal profile for an existing polishing pad of known characteristics, observing how variations in polishing surface densities and/or coefficients of friction affect that material removal profile, and then mapping the polishing surface coefficients of friction and density profiles to the subject polishing pad layout.

RELATED APPLICATIONS

This is a Continuation-in-Part of U.S. patent application Ser. No.11/697,622, filed 6 Apr. 2007, which application is assigned to theassignee of the present invention and incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of chemical mechanicalplanarization (CMP) and relates specifically to a CMP polishing padhaving a non-planar material removal profiles and methods for using suchpads.

BACKGROUND

In modern integrated circuit (IC) fabrication, layers of material areapplied to embedded structures previously formed on semiconductorwafers. Chemical mechanical planarization (CMP) is an abrasive processused to remove excess material from these layers and polish theresulting surface to achieve a desired structure and material profile.CMP may be performed on both oxides and metals and generally involvesthe use of chemical slurries applied in conjunction with a polishing padthat is put in motion relative to the wafer (e.g., rotational motionrelative to the wafer). The resulting smooth flat surface is necessaryto maintain proper photolithographic depth of focus for subsequent waferprocessing steps and to ensure that metal interconnects are not deformedover underlying features on the wafer. Damascene processing requiresmetal, such as tungsten or copper, to be removed from a top surface of adielectric to define interconnect structures, using CMP.

Polishing pads used in CMP processes are typically made of urethanes,either in cast form and filled with micro-porous elements, or fromnon-woven felt coated with polyurethanes. The polishing surface of apolishing pad is typically a single, continuous sheet of material, whichmay be grooved or perforated to facilitate slurry distribution acrossthe surface. During polishing operations the polishing pad is rotatedwhile contacting the wafer, which is also rotated, with the slurry layerdisposed between the pad and the wafer, thus affecting polishing.

One consequence of the use of conventional polishing pads in polishingprocesses is that as pad moves relative to the wafer, sudden changes inpad compression will be experienced. These variations will give rise to“edge effects” on the wafer, causing material removal rates at the edgeof the wafer to be different from those experienced at other pointsacross the diameter of the wafer.

Further, conventional polishing pads have no inherent ability tomodulate their material removal profile across the diameter of a wafer.Yet, in advanced wafer processing operations there are multiple filmsbeing deposited, each of which have specific deposition profiles. Forexample, electroplated copper films tend to be thickest at the edge of awafer, while some dielectric films tend to have a smooth “M” or“W”-shaped profile across a diameter of a wafer. In cases of criticalprocess modules, such as copper and STI polishing, the use ofconventional polishing pads can lead to over-polishing of some areas inorder to ensure that other areas of the wafer receive adequatepolishing. In advanced technology nodes, the margin for suchover-polishing (i.e., the ability to tolerate such conditions and stillhave an acceptable wafer result from the process) is shrinking rapidlyand in some cases allows for less than 5% polish time. This leads toloss of performance or, worse, loss of yield for some parts. There istherefore a need to provide means for tuning the removal profile of apolishing pad in polishing processes to minimize the need forover-polishing across the diameter of a wafer.

SUMMARY OF THE INVENTION

In one embodiment, a polishing pad includes a plurality of polishingsurfaces, a first group of the polishing surfaces made of a firstmaterial having a first coefficient of friction and a second group ofthe polishing surfaces made of a second material having a secondcoefficient of friction. The first and second groups of polishingsurfaces may be arranged over the polishing pad so as to provide anon-planar material removal profile. The first and second groups ofpolishing surfaces may be arranged to provide (1) an edge fast materialremoval profile; (2) an edge slow removal profile; (3) a center fastremoval profile; or (4) a center slow removal profile.

The polishing surfaces of the polishing pad may, in some cases, beindividual polishing elements, each polishing element supported by anunderlying compressible foam layer and oriented with a long axis normalto a plane defined by the underlying compressible foam layer. Thepolishing elements may be spaced apart from one another so thatdisplacement of one polishing element along its long axis in a directionnormal to the plane defined by the underlying foam layer does notmaterially affect displacement of adjacent ones of the polishingelements. In some cases, the polishing elements of the first group maybe made of polyurethane and the polishing elements of the second groupsmay be made of polyoxymethylene.

In varying embodiments of the invention, the first and second groups ofpolishing surfaces may be arranged in different densities over thepolishing pad. In some cases, the different densities vary by radialdistance of the polishing surfaces from a center of the polishing pad.In other cases, the different densities vary orthogonally across thepolishing pad. In still other cases, the polishing surface densityvariations are implemented by varying groove pitch and/or groove sizeover the polishing pad. In still further cases, however, overalldensities of polishing surfaces per unit area of the polishing pad isuniform for the polishing pad.

In some instances, the first and second groups of polishing surfaces maybe laid out in uniform radial arrangement over the entire polishing pad.Each polishing surface of the first and second groups may have a commonsize and/or common shape or different sizes/shapes. Where the polishingsurfaces are individual polishing elements, they may be laid out in anorthogonal arrangement where inter-polishing element spacing is definedby a pitch in each of two dimensions. This inter-polishing element pitchmay vary in either or both of these dimensions. Further, where thepolishing surfaces are individual polishing elements they may be laidout in a radial pattern such that polishing elements at different radialdistances from a center of the polishing pad have differentinter-polishing element spacing.

Further embodiments of the invention provide for polishing a workpieceusing a polishing pad configured in any of the above-described fashions.

Moreover, designing a layout of polishing surfaces for a polishing padto achieve a desired material removal profile for the polishing pad maybe done by (a) determining a first material removal profile of a firstpolishing pad having a known polishing surface layout, (b) applyingpolishing surface coefficient of friction variations and/or polishingsurface density variations corresponding to regions of the firstpolishing pad where changes in the first material removal profile aredesired, (c) determining what fraction of a total polishing time isspent by an area of a wafer of interest in the regions of the first pad,(d) determining, based on these fractional times, correspondingvariations in the first material removal profile at pad-wafer locationsof interest, and (e) determining the new pad layout to achieve thedesired material removal profile by mapping new polishing surfacecoefficients of friction and density profiles to the subject polishingpad layout.

These and further embodiments of the invention are discussed in detailbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the figures of the accompanying drawings, in which:

FIG. 1 illustrates an example of a polishing pad having individualpolishing elements, densities of which across the pad may be variedaccording to the present invention;

FIG. 2A shows a polishing pad having polishing elements laid out invarying densities along circumferences of different radii in accordancewith an embodiment of the present invention.

FIG. 2B shows a polishing pad in which polishing elements are laid outin an orthogonal arrangement and the inter-polishing element spacing isdefined by a pitch in two dimensions.

FIG. 2C illustrates a polishing pad similar to that shown in FIG. 2B,with variations in polishing pad pitches in each dimension.

FIG. 3 shows a polishing pad with a wafer overlaid thereon (e.g., as maybe the case during wafer polishing operations) and containing polishingelements laid out in a fixed radial density profile and in whichpolishing elements of two different types are present.

DETAILED DESCRIPTION

Described below is a polishing pad suitable for use in CMP and otherapplications, which pad is configured to provide a tunable, materialremoval profile across a workpiece being polished. Often, for examplewith CMP processes, the workpiece will be a semiconductor wafer (eithera bare wafer or one that has already undergone one or more processingsteps in a fabrication process), but this is not necessarily so. Otherworkpieces that are subject to polishing operations may also benefitfrom the use of the polishing pads configured in accordance with thepresent invention. Therefore, although the present polishing pad will bediscussed in the context of semiconductor polishing operations (inparticular CMP), it should be recognized that the present polishing padmay find use in many other applications.

In various embodiments, the present polishing pad consists of polishingelements which can be arranged over the surface of the pad in differentconfigurations. For example the polishing elements may be arranged in afixed area density configuration or a fixed radial densityconfiguration. These different layouts offer specific advantages intheir ability to deliver uniform material removal profiles, depending onthe motion of the pad relative to the wafer. A fixed area densityprofile may be more advantageous for linear or orbital relative motion,while a fixed radial density profile maybe more suitable when relativerotary motion is employed. Of course, other polishing element layoutsmay also be used.

The above-cited U.S. patent application Ser. No. 11/697,622 described apolishing pad that includes a number of individual polishing elements.Such a pad 100 is illustrated in cross-section in FIG. 1. Each polishingelement 102 is supported by an underlying compressible foam layer 104and extends vertically through holes in a guide plate 106 affixed tothat underlying foam layer. The polishing elements are maintained sothat they are approximately normal with respect to a plane defined bythe underlying foam layer and are spaced apart from one another so thatdisplacement of one polishing element along its long axis in a directionnormal to the plane defined by the underlying foam layer does notmaterially affect displacement of adjacent ones of the polishingelements.

The polishing pad described in the '622 application enabled very uniformpressure to be exerted onto a wafer surface during polishing operations.This translates to a very uniform material removal profile in which edgeeffects typically associated with conventional polishing pads aresignificantly reduced (or in some cases eliminated altogether).Nevertheless, in some instances it is desirable to tune the polishingprofile of the polishing pad in order to effect different materialremoval profiles. Hence, in embodiments of the present invention, theindividual polishing elements may be organized in many differentarrangements (e.g., different density profiles that have more or fewerpads per unit area across the face of the pad) and/or may be made fromdifferent materials (e.g., having different coefficients of friction forthe material being polished) in order to effect such varying materialremoval profiles.

In one embodiment, a polishing pad configured to effect non-planarmaterial removal across a wafer during polishing is made up of a numberof individual polishing elements, each supported by an underlyingcompressible foam layer and maintained in vertical orientation withrespect to said foam underlayer by a guide plate having holes throughwhich the individual polishing elements protrude. The nominal diameterof an individual polishing element may be approximately 0.25 inches andthe nominal height of an individual polishing element may beapproximately 0.160 inches. The compressible foam underlayer may benominally 0.060 inches thick.

The polishing pad may include polishing elements made of two (or more)different materials, each having a different coefficient of friction forthe film or material being polished. The polishing elements may bearranged such that the polishing elements with lower coefficient offriction, such as Delrin™ (polyoxymethylene), alternate in a desiredfashion or density with polishing elements having higher coefficients offriction (e.g., polyurethane polishing elements). The ratio of onevariety of polishing element to another may be varied to achievedifferent material removal rate performance.

The nominal coefficient of friction for polyurethane against an SiO₂surface is 0.45, whereas the coefficient of friction for Delrin undersimilar conditions is 0.15. Therefore, the present polishing pad isexpected to have a much lower material removal contribution frompolishing elements made of Delrin than for other polishing elements,thus leading to a lower overall material removal rate than for a padhaving only non-Delrin polishing elements. During polishing, an area ofa wafer in contact with a section of the pad that contains both Delrinand non-Delrin (e.g., polyurethane) polishing elements will have lowerrate of material removal than an area of the wafer in contact with onlynon-Delrin (e.g., polyurethane) polishing elements.

In addition, embodiments of the present polishing pad may have polishingelements arranged to provide a fixed area density of polishing elementsor a fixed radial density of polishing elements (either in total or perpolishing element material). These different layouts offer specificadvantages in their ability to deliver differing material removal ratesprofiles, depending on the relative motion of the polishing pad to thewafer. A fixed area density profile may be more advantageous for linearor orbital relative motion, while a fixed radial density profile maybemore suitable when relative rotary motion is employed. FIG. 2A shows apolishing pad 200 having an arrangement of polishing elements 202 inwhich the polishing elements 202 are laid along circumferences ofdifferent radii (marked as R1, R2, R3 and R4). FIG. 2B shows a polishingpad 204 in which the polishing elements 202 are laid out in anorthogonal arrangement where the inter-polishing element spacing isdefined by a pitch in both X and Y directions. As shown in FIG. 2C,either or both pitches (X, Y) may be varied across a diameter of a wafer206 to provide two or more pitches (X1, X2; Y1, Y2) in each dimension.

FIG. 3 shows a polishing pad 300 with a wafer 302 overlaid on the pad(e.g., as may be the case during wafer polishing operations). The pad300 contains polishing elements 304A, 304B laid out in a fixed radialdensity profile in which polishing elements of two different types arepresent. That is, polishing elements 304A are made of a differentmaterial than polishing elements 304B. For example, polishing elements304A may be made of polyurethane while polishing elements 304B may bemade of material having a different coefficient of friction with respectto a surface of wafer 302 (or a film on wafer 302) presently beingpolished. Note that the polishing elements of different materials may beused with any of the configurations shown in FIGS. 2A-2C.

In an embodiment of the invention, a determination as to which polishingelement material combination and/or layout to use in order to achieve adesired film removal profile may be accomplished. For example, thematerial removal rate for a given substrate or film will be proportionalto the area of the polishing pad in contact with that substrate or film.This will, in turn, be affected by the density of polishing elements perunit area of the pad. The effective polishing element density of a padmay be varied by changing the pitch and/or size of polishing elements inany given area thereof. Therefore, assuming that as the wafer rotatesrelative to the pad, and the wafer traverses the entire radius of thepad during a polishing operation, then by varying the layout density ofpolishing elements (hence the effective contact area of the pad) andcalculating the time spent by the wafer in areas of different polishingelement densities across the entire wafer, a removal profile for thepad/substrate (or pad/film) combination can be predicted.

Conversely, by knowing the removal profile for a given polishing elementmaterial/density layout, a polishing pad can be constructed to achieve adesired layout/material removal profile. Of course, the same processescan be applied to polishing pads using polishing elements of differentcoefficients of friction (or other polishing element characteristicsthat affect material removal rates) in lieu of or in addition to varyingpolishing element densities. Stated differently, the above-describedprocesses can be used to develop custom tailored pads for givensubstrates or films and selected polishing element materials to providedesired material removal rates.

In various embodiments of the invention then, the polishing elementdensity of a polishing pad may be varied over areas from 10 mm² to 1000mm², and preferably from 30 mm² to 250 mm². Such polishing elementdensities may be varied radially across the pad surface or orthogonallyacross the pad surface. The polishing element density range across thepad may be between 20%-80% of the entire pad surface, and preferablybetween 35-60% of the pad surface. The polishing element densityvariation may be achieved by varying polishing element pitch and,optionally, the size of the discrete polishing element surfaces whichcontact the substrate or film to be polished. Alternatively, polishingelement density variation may achieved by varying groove pitch and/orgroove size, as in the case of continuous polishing surfaces forcontinuous layer polishing pads.

In a further embodiment of the invention, desired material removal ratesfor a polishing pad may be obtained by varying polishing surfacecoefficients of friction over an area of the polishing pad. Such areamay vary between 10 mm²-1000 mm², and preferably between 30 mm²-250 mm².Coefficients of friction of the polishing surfaces may be varied acrossthe pad in the range of 0.1-0.8, and preferably 0.3-0.6. This variationin the coefficients of friction may be achieved by varying the materialof which the subject polishing surface is made. In some cases, thepolishing surface may be made up of multiple individual polishingelement surfaces, while in other cases a continuous polishing surfacemay be used.

A polishing pad configured in accordance with the above practices maytherefore contain a plurality of polishing surface materials, eachhaving a different coefficient of friction, and arranged so as toachieve non-planar removal profile. For example, the materials may bearranged to achieve edge fast removal profile, an edge slow removalprofile, a center fast removal profile, or a center slow removalprofile. In one instance, a polishing pad contains a plurality ofpolishing surface materials each having different coefficient offriction and the materials of different coefficients of friction arearranged according to different layout densities over the surface of thepad to achieve the non planar removal profile.

For example, some of the plurality of polishing surface materials (whichmay be individual polishing element surfaces or areas of a continuouspolishing surface) may be made from polyurethane and others of theplurality of polishing surfaces are made of Delrin. Each of therespective polishing surfaces may be arranged in a radial manner acrossthe pad such that Delrin polishing surfaces make up 5-50% density inlocations of the total polishing surface of the pad corresponding toareas requiring low material removal rates. The pad may be used inpolishing operations such that it contacts a wafer or other substrate(or a film thereon), optionally in the presence of slurry, such that thewafer areas requiring low material removal rates are in contact with padareas containing Delrin polishing surfaces in the desired density. Insome instances, the overall density of polishing surfaces may be uniformper unit area of the polishing pad. Alternatively, or in addition, thepolishing surfaces may be laid out in a uniform radial arrangement.Optionally, both the Delrin and polyurethane polishing surfaces may havea common shape and size, but in other cases the respective polishingsurfaces may have different sizes and shapes.

A polishing pad polishing surface layout designed to achieve a desiredmaterial removal profile may be achieved by (a) determining a removalprofile of a known polishing pad polishing surface layout, (b) applyingpolishing surface coefficient of friction variations and/or densityvariations corresponding to regions of the pad where changes in materialremoval rates are desired, (c) determining what fraction of the totalpolishing time is spent by an area of the wafer of interest in theregion of modified polishing surface coefficients of friction/densitiesof the pad, (d) determining, based on these fractional times,corresponding variations in material removal rates at a pad-waferlocations of interest, and (e) determining the new pad layout to achievethe desired material removal profile by mapping new polishing surfacecoefficients of friction and density profiles to the subject polishingpad layout.

Thus, a polishing pad suitable for use in CMP and other applications,which pad is configured to provide a tunable, material removal profileacross a workpiece has been described. In the above description, anumber of specifically illustrated embodiments were discussed in orderto better explain the present invention, however, these examples shouldnot be read to limit the scope of the invention. Instead, the inventionshould be measured only in terms of the following claims.

1. A polishing pad, comprising a plurality of polishing surfaces, afirst group of the polishing surfaces made of a first material having afirst coefficient of friction and a second group of the polishingsurfaces made of a second material having a second coefficient offriction, the first and second groups of polishing surfaces arrangedover the polishing pad so as to provide a non-planar material removalprofile.
 2. The polishing pad of claim 1, wherein the first and secondgroups of polishing surfaces are arranged to provide an edge fastmaterial removal profile.
 3. The polishing pad of claim 1, wherein thefirst and second groups of polishing surfaces are arranged to provide anedge slow removal profile.
 4. The polishing pad of claim 1, wherein thefirst and second groups of polishing surfaces are arranged to provide acenter fast removal profile.
 5. The polishing pad of claim 1 wherein thefirst and second groups of polishing surfaces are arranged to provide acenter slow removal profile.
 6. The polishing pad of claim 1, whereinthe polishing surfaces comprise individual polishing elements, eachpolishing element supported by an underlying compressible foam layer andoriented with a long axis normal to a plane defined by the underlyingcompressible foam layer.
 7. The polishing pad of claim 6, wherein thepolishing elements are spaced apart from one another so thatdisplacement of one polishing element along its long axis in a directionnormal to the plane defined by the underlying foam layer does notmaterially affect displacement of adjacent ones of the polishingelements.
 8. The polishing pad of claim 1, wherein the polishingelements of the first group are made of polyurethane and the polishingelements of the second groups are made of polyoxymethylene.
 9. Thepolishing pad of claim 1, wherein the first and second groups ofpolishing surfaces are arranged in different densities over thepolishing pad.
 10. The polishing pad of claim 9, wherein the differentdensities vary by radial distance of the polishing surfaces from acenter of the polishing pad.
 11. The polishing pad of claim 9, whereinthe different densities vary orthogonally across the polishing pad. 12.The polishing pad of claim 9, wherein polishing surface densityvariations are implemented by varying groove pitch over the polishingpad.
 13. The polishing pad of claim 9, wherein polishing surface densityvariations are implemented by varying groove size over the polishingpad.
 14. The polishing pad of claim 1, wherein overall densities ofpolishing surfaces per unit area of the polishing pad is uniform for thepolishing pad.
 15. The polishing pad of claim 1, wherein the first andsecond groups of polishing surfaces are laid out in uniform radialarrangement over the entire polishing pad.
 16. The polishing pad ofclaim 1, wherein each polishing surface of the first and second groupshas a common size.
 17. The polishing pad of claim 1, wherein eachpolishing surface of the first and second groups has a common shape. 18.The polishing pad of claim 1, wherein the polishing surfaces compriseindividual polishing elements laid out in an orthogonal arrangementwhere inter-polishing element spacing is defined by a pitch in each oftwo dimensions.
 19. The polishing pad of claim 18, wherein theinter-polishing element pitch in at least one of the two dimensionsvaries across the polishing pad.
 20. The polishing pad of claim 18,wherein the inter-polishing element pitch in both of the two dimensionsvaries across the polishing pad.
 21. The polishing pad of claim 1,wherein the polishing surfaces comprise individual polishing elementslaid out in a radial pattern such that polishing elements at differentradial distances from a center of the polishing pad have differentinter-polishing element spacing.
 22. A method, comprising polishing aworkpiece using a polishing pad defined by any one of the precedingclaims.
 23. A method, comprising designing a layout of polishingsurfaces for a polishing pad to achieve a desired material removalprofile for the polishing pad by (a) determining a first materialremoval profile of a first polishing pad having a known polishingsurface layout, (b) applying polishing surface coefficient of frictionvariations and/or polishing surface density variations corresponding toregions of the first polishing pad where changes in the first materialremoval profile are desired, (c) determining what fraction of a totalpolishing time is spent by an area of a wafer of interest in the regionsof the first pad, (d) determining, based on these fractional times,corresponding variations in the first material removal profile atpad-wafer locations of interest, and (e) determining the new pad layoutto achieve the desired material removal profile by mapping new polishingsurface coefficients of friction and density profiles to the subjectpolishing pad layout.